1. Field of the Invention
The invention relates to a method for the transmission of data via a bus network to which a plurality of subscribers are connected, and also apparatuses and bus networks adapted for carrying out the method.
2. Description of the Related Art
In particular for applications from the field of safety technology, the recording of the data stream transmitted via a bus network is an expedient property for example in order to have access to application-specific safety-orientated input and output data in a simple manner and consequently to detect errors as early as possible and to correspondingly minimize permissible maximum reaction times.
Based on the ring system of a standard Interbus according to DIN E 19258, such data recording is appropriate at the present time only directly in or downstream of the master, since it is only at this location that all the input or output information is present for evaluation.
Furthermore, a point-to-point-based transmission method has the disadvantage that many data that are not required for the actual function, that is to say in particular the so-called overhead data, have to be transmitted individually for each subscriber, as a result of which the effectiveness of the transmission is reduced overall, which has an adverse effect on the reaction times, that is to say essentially on the time between the receipt of input data, e.g. via sensors of input components of the connected subscribers, and corresponding output data, e.g. for actuators of output components of the connected subscribers, in the safety system.
In the case of the Interbus, safety-relevant input data are transmitted e.g. by a central unit in the Interbus, which central unit is integrated in the master or else may be remote from the latter, concomitantly reading and evaluating safety-orientated data.
In contrast to the described transmission path of the input data into the central controller or a monitoring device, the outputting of safety-orientated signals imposes particular requirements, however. On the one hand, said signals are significantly smaller in number compared with the input data; on the other hand, however, the safety-orientated data that are output generally have to be able to be transmitted as rapidly as possible.
In existing safety systems, therefore, data are conventionally interchanged according to the call/response principle, in which case in present-day customary solutions a central unit or a monitoring unit transmits the safety data by the call/response method to the connected safety-orientated subscribers. However, an essential disadvantage in this case resides in the need—demonstrated above—to transmit, for each subscriber, numerous so-called overhead data that are actually redundant.
In some instances, furthermore, manipulations, for example in the Interbus data frame, are even necessary under some circumstances, the data having to be buffer-stored for such manipulations and a delay thus appearing, which leads to a further significant lengthening of the reaction time.
Other transmission systems, in particular from the field of IT, already have apparatuses for transmitting broadcast messages.
By way of example, reference shall be made in this respect to networks and communication techniques which have a common transmission medium with multiple access and dynamic assignment of the transmission medium. With the aid of the respective multiple access methods, it is possible besides pure point-to-point or unicast transmission, in which data are transmitted to a single receiver, also for information to be transmitted simultaneously to a plurality of subscribers by means of a multicast and a broadcast transmission, so that in this case the proportion of overhead data is reduced compared with a multiplicity of separate unicast transmissions.
German patent specification 199 35 490 discloses in this regard a method and also a control and data transmission system for providing a corresponding communication between a plurality of subscribers and a network which has at least one multiple access channel assigned dynamically. In this case, one of the subscribers is designed as a master and the other subscribers are designed as slaves, each subscriber being assigned a standard multiple access protocol with the capability of broadcast data transmission and a master/slave protocol entity which is superposed on the standard multiple access protocol and controls the fetching of corresponding services of the standard multiple access protocol for executing a master/slave transmission method. In accordance with the disclosure, each communication cycle comprises the steps of transmitting a synchronization message from the master to the participating slaves, at least one item of information that identifies the communication cycle being transmitted in the synchronization message, transmitting a call message from the master to a participating slave by means of a unicast call, and transmitting a response message from the slave to the master and to the rest of the participating slaves in the broadcast mode.
In accordance with the disclosure of the above patent specification, the master/slave connection method that is customary for field buses is consequently mapped onto the protocols currently to be encountered in LAN networks.
Consequently, even in existing safety systems which intrinsically already have apparatuses for transmitting broadcast messages, at least output data are conventionally transmitted according to the call/response principle.